1. Field of the Invention
The present invention relates to plastic pallets and, more particularly, to plastic pallets having flame retardant properties.
2. Description of the Related Art
The use of plastic pallets is known and such pallets are generally required to meet the flame retardant standards such as UL 2335 and FM4996. These standards both involve large scale burn tests that are conducted by testing actual production parts. For example, such a test may include burning an array of 6 columns of 26 pallet stacks. Such tests involve measuring the time required for sprinkler set-off, the number and time of flame breaches out of the sides of the pallet stack, time for total pallet burn extinction, and heat generated during the pallet burn test. It is noted that the terms plastic pallet and polymeric pallet are used interchangeably herein.
Generally, the process for manufacturing plastic pallets includes forming a top deck from thermoformed extruded sheet material and forming the bottom deck from an injection molded section. It is common for both the top deck and bottom deck to utilize high density polyethylene (“HDPE”) and a fire retardant (“FR”) system along with a standard color concentrate added during the extrusion or molding process. For the extruded/thermoformed sheet component, the HDPE used is typically what is referred to as high molecular weight polyethylene which is by definition not only a high density polyethylene (0.95 density or above) but also a very high molecular weight/very high viscosity/very low melt flow HDPE. The typical melt flow for the extruded sheet component is in the range of 8-25 under High Load Melt Index (“HLMI”) conditions (190 C/21.6 kg load when measured under the ASTM D1238 method). For the injection molded component the melt flow tends to range between 4 and 10 with 6 to 8 being preferred (the injection molded component is typically formed out of standard HDPE, not a high molecular weight polyethylene, and these melt flow ranges are measured under the standard polyethylene conditions of 190 C/2.16 kg load which corresponds to condition E in ASTM D1238 not HLMI conditions. The separately extruded/thermoformed sheet and injection molded component are commonly heat welded together to form the integral plastic pallet, and steel reinforcement beams are often added to increase rigidity and strength as needed.
Several FR are known in the art. Such known FR include brominated organic compounds, more particularly, polybrominated diphenyl oxides such as pentabromodiphenyl oxide (“PENTA”), octabromodiphenyl oxide (“OCTA”) and decabromodiphenyl oxide (“DECA”). PENTA and OCTA are effective and were commonly used in the past but in recent years have been phased out of use by regulations. As the use of PENTA and OCTA was discontinued, DECA became more popular as it found increased usage as a replacement for PENTA and OCTA.
DECA quickly became one of the most widely used and cost-effective brominated FR used with plastic pallets. When used with polythylene pallets, it has been shown to generally pass the UL and FM standards when used at approximately 4 to 5% (by weight) in conjunction with 2 to 2.5% (by weight) antimony trioxide. The efficacy of DECA, however, can vary depending upon the amount of DECA used and type of polyethylene used.
The use of DECA is not without downsides. For example, it often migrates to the surface of the plastic parts in a process commonly referred to as “blooming.” If the blooming is bad enough it can affect the ability of the two components (extruded/thermoformed sheet and molded component) to form a sufficiently strong weld. The UV stability of DECA is also known to be poor and can result in the loss of color, gloss and other physical properties of the plastic parts containing the DECA. It is also subject to plate out and deposition onto process equipment especially during a sheet extrusion process. While deposition on process equipment does not directly impact performance, it does cause additional equipment maintenance expenses.
Environmental concerns related to the bioaccumulation and environmental persistence of DECA has recently led to increased regulation by EPA and OSHA. As a result, DECA is in the process of being phased out over the next several years (with a gradual reduction of usage every year by 25% or more by region) until it is banned for usage by the end of 2013 (except for aerospace and certain military applications if no effective alternative is available).
A fire retardant alternative to DECA that minimizes or eliminates blooming while still allowing plastic pallets formed with HDPE blends to satisfy the UL 2335 and FM 4996 standards is desirable.